Field of the Invention
This invention relates to semiconductor devices, and in particular to light emitting devices capable of wire bond free fabrication and operation.
Description of the Related Art
Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light from the active layer is emitted from all surfaces of the LED.
There has been a great deal of recent interest in LEDs formed of Group-III nitride based material systems because of their unique combination of material characteristics including high breakdown fields, wide bandgaps (3.36 eV for GaN at room temperature), large conduction band offset, and high saturated electron drift velocity. The efficient extraction of light from LEDs is a major concern in the fabrication of high efficiency LEDs. For conventional LEDs with a single out-coupling surface, the external quantum efficiency is limited by total internal reflection (TIR) of light from the LED's emission region that passes through the substrate. TIR can be caused by the difference in the refractive index between the LED semiconductor and surrounding ambient, as predicted by Snell's Law. This difference results in a small escape cone from which light rays from the active area can transmit from the LED surfaces into the surrounding material and ultimately escape from the LED package.
Different approaches have been developed to reduce TIR and improve overall light extraction, with one of the more popular being surface texturing. Surface texturing increases the escape probability of the light by providing a varying surface that allows photons multiple opportunities to find an escape cone. Light that does not find an escape cone continues to experience TIR, and reflects off the textured surface at different angles until it finds an escape cone. The benefits of surface texturing have been discussed in several articles. [See Windisch et al., Impact of Texture-Enhanced Transmission on High-Efficiency Surface Textured Light Emitting Diodes, Appl. Phys. Lett., Vol. 79, No. 15, October 2001, Pgs. 2316-2317; Schnitzer et al. 30% External Quantum Efficiency From Surface Textured, Thin Film Light Emitting Diodes, Appl. Phys. Lett., Vol. 64, No. 16, October 1993, Pgs. 2174-2176; Windisch et al. Light Extraction Mechanisms in High-Efficiency Surface Textured Light Emitting Diodes, IEEE Journal on Selected Topics in Quantum Electronics, Vol. 8, No. 2, March/April 2002, Pgs. 248-255; Streubel et al. High Brightness AlGaNInP Light Emitting Diodes, IEEE Journal on Selected Topics in Quantum Electronics, Vol. 8, No. March/April 2002].
U.S. Pat. No. 6,657,236, assigned to Cree Inc., discloses structures for enhancing light extraction in LEDs through the use of internal and external optical elements formed in an array. The optical elements have many different shapes, such as hemispheres and pyramids, and may be located on the surface of, or within, various layers of the LED. The elements provide surfaces from which light refracts or scatters. Also, a reflective material may be used to coat one or more of the layers of the device to enhance light extraction by reflecting light emitted from the active layers away from the substrate or other photon absorbing materials.
Another method used to fabricate more efficient semiconductor devices is called flip-chip mounting. Flip-chip mounting of LEDs involves mounting the LED onto a submount substrate-side up. Light is then extracted and emitted through the transparent substrate, or the substrate may be removed altogether. Flip-chip mounting is an especially desirable technique for mounting SiC-based LEDs. Since SiC has a higher index of refraction than GaN, light generated in the active region does not internally reflect (i.e. reflect back into the GaN-based layers) at the GaN/SiC interface. Flip-chip mounting of SiC-based LEDs offers improved light extraction when employing certain chip-shaping techniques known in the art. Flip-chip packaging of SiC LEDs has other benefits as well, such as improved heat extraction/dissipation, which may be desirable depending on the particular application for the chip.
Significant effort has been invested in developing a white light LED. Conventional LEDs cannot generate white light, i.e., a broad spectrum, directly from their active layers. Light from a blue emitting LED has been converted to white light by surrounding the LED with a yellow emitting phosphor, polymer or dye, with a typical phosphor being cerium-doped yttrium aluminum garnet (Ce:YAG). [See Nichia Corp. white LED, Part No. NSPW300BS, NSPW312BS, etc.; See also U.S. Pat. No. 5,959,316 to Lowrey, “Multiple Encapsulation of Phosphor-LED Devices”]. The surrounding phosphor material “downconverts” the wavelength of some of the blue light, changing its color to yellow. Some of the blue light passes through the phosphor without being changed while a substantial portion of the light is downconverted to yellow. The LED emits both blue and yellow light, which combine to provide a white light. In another approach light from a violet or ultraviolet emitting LED has been converted to white light by surrounding the LED with multicolor phosphors or dyes.
LED devices are often described as having a vertical geometry or a lateral geometry as shown in FIGS. 1 and 2, respectively. Both configurations are known in the art. Vertical geometry devices typically feature p-contact and n-contact electrodes on opposite surfaces of the device. Charge carriers move vertically through the semiconductor layers in response to a bias. Lateral geometry devices are usually arranged with a split-level electrode configuration with both electrodes on the top surface of layers on different levels of the device. Thus, the electrodes do not share a common surface but rather a common top-side orientation with respect to the layers on which they are disposed. Charge carriers move laterally through the semiconductor layers for at least a portion of the current path in response to a bias. Several variations of these common geometries are known and used in the art.
FIG. 1 illustrates a vertical geometry nitride LED 100 that is known and used in the art. An active region 102 is interposed between p-type layer 104 and n-type layer 106. The semiconductor layers are grown on a conductive substrate 108. A thin semi-transparent current spreading contact 110 covers most or all of the p-type layer. A bias is applied to the device 100 using electrode 112 and substrate 108. Electrode 112 is connected to an outside voltage source (not shown) via wire 114. The substrate 108 may be connected to the voltage source with solder bumps, pads or wires on the bottom side of the wafer 108. Phosphor layer 118 covers all the surfaces of the device with wire 114 protruding through the phosphor layer 118.
In response to an applied bias, current and charge carriers move through the device 100 vertically with respect to the semiconductor surfaces. Radiative recombination occurs in the active region 102 and light is emitted. Some of the emitted light has its wavelength downconverted in the phosphor layer, resulting in a desired emission spectrum.
FIG. 2a illustrates an LED device 200 having a split-level lateral geometry that is known and used in the art. An active region 202 is interposed between p-type layer 204 and n-type layer 206. The semiconductor layers are grown on a substrate 208. A thin semi-transparent current spreading contact 210 covers most or all of the p-type layer. A bias is applied to the device 200 using p-contact electrode 212 and n-contact electrode 214. Wires 216, 218 provide connections to an outside voltage source (not shown). A phosphor layer 220 covers all the surfaces of the device with wires 216, 218 protruding through the phosphor layer 220.
The bias is applied to the device 200 through electrodes 212, 214. Current and charge carriers move laterally through the device between the electrodes 212, 214. A percentage of the carriers recombine in the active region 202, causing light to be emitted. Some of the emitted light has its wavelength downconverted in the phosphor layer 220, enabling the device to emit light with desired wavelength spectrum.
FIG. 2b illustrates a known LED device 250 similar to the device 200 shown in FIG. 2a. The device 250 features the flip-chip configuration with the growth substrate 252 disposed above the n-type layer 254, the active region 256, and the p-type layer 258. After the semiconductor layers 254, 256, 258 are grown on the growth substrate 252, the device 250 is flipped and mounted to a surface. Thus, the device emits light through the growth substrate. This configuration requires a transparent substrate so that the light may escape the device primarily through the top surface. A phosphor layer 260 coats the entire device and downconverts a portion of the light emitted from the active region 258. An n-contact electrode 262 and a reflective p-contact electrode 264 are disposed on the bottom side of the device 250 to provide the necessary bias for radiative recombination. The device 250 emits light from the active region 256, most of which is emitted out the top surface of the device 250. A portion of the light is absorbed and/or back-scattered by the growth substrate 252 before it is emitted.
FIG. 3 depicts a typical flip-chip LED device 300 having a vertical geometry configuration that is known in the art. Oppositely doped n-type layer 302 and p-type layer 304 sandwich the active region 306. A reflective element 308, such as a mirror, is shown bonded to a carrier wafer 310 with a metal bond 312. In this particular configuration the LED device 300 has been flip-chip mounted, and the reflective element 308 is adjacent to p-type layer 304. The n-type layer 302, the p-type layer 304 and active region 306 are grown on a growth substrate (not shown) that is subsequently removed. The exposed surface of the n-type layer 302 is textured or roughened to improve light extraction. A layer of conversion material 314, such as phosphor for example, can be disposed over an n-pad 316 that provides a surface to which a wire 318 can be bonded. The wire 318 connects the device to an external voltage/current source (not shown). In this particular device 300 because n-type layer 302, p-type layer 304 and active region 306 are very thin and the growth substrate is removed, the phosphor layer only needs to coat the top surface.
One disadvantage inherent to all of these exemplary configurations is that their design prevents package level components such as, for example, a phosphor layer or an encapsulation structure from being applied until after the device is singulated and mounted in a conventional LED package. In some cases the constraint is caused by the need to connect the device to an outside voltage source using a wire bond or other similar means of connection. In other cases the constraint is caused by the need to coat the sides of the substrate with phosphor to prevent too much blue light from escaping without being downconverted.